The present invention relates to a controller for a permanent magnet field motor that has a first rotor and a second rotor concentrically provided around a rotating shaft and a phase changing device for changing an angle of relative displacement in a circumferential direction between the first rotor and the second rotor and that is provided as a power source for driving driven wheels of an all-wheel-drive vehicle having main driving wheels and driven wheels.
Hitherto, a motor has been known, which has a first rotor and a second rotor concentrically provided around a rotating shaft of a permanent magnet field rotary motor and which performs a field weakening control operation by changing a phase difference between the first rotor and the second rotor according to a rotational speed thereof or to the speed of a rotating field generated in a stator (see, e.g., Patent Document 1).
In a case where the motor described in Patent Document 1 controls the phase difference between the first rotor and the second rotor according to the rotational speed thereof, an angle of relative displacement in the circumferential direction therebetween is changed via a member displaced along a radial direction of the motor by the action of a centrifugal force. Further, in a case where the phase difference therebetween is controlled according to the speed of the rotating field generated in each of the rotors, the angle of relative displacement in the circumferential direction therebetween is changed by supplying control electric current to the winding of the stator so as to change the speed of the rotating field in a state in which each of the rotors maintains the rotational speed thereof by inertia.    [Patent Document 1] JP-A-2002-204541    [Patent Document 2] JP-A-2007-236049    [Patent Document 3] JP-A-2007-259549
Some four-wheel-drive vehicles are such that one of front and rear pairs of wheels serves as a pair of main driving wheels and is driven by an internal combustion engine and/or a motor, and that the other of the front and rear pairs of wheels serves as a pair of driven wheels and is driven by another motor. Such a vehicle can realize stable and powerful running, as compared with two-wheel-drive vehicles, by simultaneously driving the four wheels. Incidentally, a driving force required by a vehicle while the vehicle cruises is lower than that required while the vehicle is accelerated. Thus, the four-wheel-drive vehicle performs running by appropriately selecting one of a four-wheel drive mode, in which the vehicle performs running by driving both of the pairs of the main driving wheels and the driven wheels, and a two-wheel drive mode, in which the vehicle performs running by driving only the pair of the main driving wheels, according to a vehicular driving condition. In case of running on the frozen road surface, the four-wheel drive mode is selected.
In a case where the aforementioned motor is used as a power source for driving a pair of driven wheels of such a four-wheel drive vehicle, when the motor is in a field strengthening state while the vehicle performs running in the two-wheel drive mode, a large drag loss is generated in the motor 1. When the vehicle performs running in the two-wheel drive mode, the first rotor and the second rotor rotate with respect to the stator in synchronization with the rotation of the driven wheels. In order to enable the first rotor and the second rotor to rotate with respect to the stator, a force being larger than a magnetic force due to a resultant magnetic flux synthesized from magnetic fluxes generated at the rotors is needed. Accordingly, in order to enable the vehicle to run in the two-wheel drive mode in a field strengthening state, a driving force exceeding the drag loss generated in the motor is required to be generated by the power source for driving the main driving wheels. Consequently, the fuel cost of the vehicle is increased.